1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting diode (LED), in which a large quantity of impurity bands is formed in a p-type contact layer such that the p-type contact layer is ohmic-contacted with an n-type transparent oxide electrode such as Indium Tin Oxide (ITO) of which the transmittance is excellent, thereby reducing contact resistance and enhancing a current spreading effect. Accordingly, it is possible to reduce an operation voltage and to increase light emission efficiency.
2. Description of the Related Art
Devices using Group III-V nitride-based compound semiconductor are widely used in various fields. For example, they are used in electronic devices such as light emitting devices (such as LEDs and laser diodes (LDs)), light receiving devices (such as solar batteries, optical sensors and the like), transistors, and power devices. LDs and LEDs using such a material are essential for developing various light sources, such as natural-color electronic display boards, traffic signals, image scanner light sources and the like, and high-density optical recording mediums.
These nitride semiconductor LEDs have such a structure that a buffer layer, an n-type nitride semiconductor layer composed of a Si-doped GaN layer, an active layer with a single quantum-well or double-quantum well structure composed of InGaN/GaN, a clad layer composed of Mg-doped p-AlGaN, and a p-type nitride semiconductor layer composed of Mg-doped g-GaN layer are sequentially laminated on a sapphire substrate.
General nitride semiconductor LEDs have already used commonly, and LDs start to be used commonly. However, in order to use the LEDs as lighting sources or outdoor displays, the power of the LEDs needs to be improved. In the LDs, a threshold value should be reduced to lengthen their lifespan. Further, the LDs need to be further improved, in order to commonly use the LDs as stable light sources such as optical pickups, DVD and the like.
In the LEDs, their caloric value is reduced by reducing their operation voltage. Then, their reliability and lifespan can be enhanced. To increase a p-cladding effect, a p-AlGaN layer is grown to increase light extraction efficiency. Because of this, a piezo electric field strongly acts, so that blue shift may increase and the operation voltage may be increased. Accordingly, a p-type contract layer composed of p+GaN is grown on the p-type nitride semiconductor layer such that an ohmic characteristic is enhanced. Then, the operation voltage is reduced.
Conventionally, transparent oxide electrodes such as ITO and the like are used for forming ohmic contact. However, since the ITO is an n-type material, it forms a Schottky junction with the p-type contact layer such that a current does not uniformly flow. Further, the contact resistance and operation voltage increase. To reduce the contact resistance, different kinds of transparent electrodes such as ClO, ZnO, and MgZnO are used, thereby reducing the operation voltage. In such a method, however, the transmittance of the transparent electrodes is reduced more that that of a transparent electrode formed of one ITO layer. Further, since thin transparent electrodes for various kinds of contacts are formed, process time is lengthened, and management is not easy.